An ionization device has been proposed (see Patent Document 1) in which a needle-shaped discharge electrode is installed in an inner space of a case, with an electrode having an orifice being placed at a position opposing the tip end of the discharge electrode and an enlarged pipe that expands in a reverse tapered state from the orifice toward the outside space, and a gas containing molecules of a predetermined substance is supplied to the inside of the case. In this ionization device, a gas containing molecules of a predetermined substance is supplied into the case, and a predetermined voltage is applied between the two electrodes to generate a discharge so that molecules of the gas inside the case are ionized and discharged from the orifice toward the outer space. Here, water is used as molecules to be ionized, and cluster ions derived from water macromolecules are generated in the outer space. In this ionization device, it is considered that an abrupt pressure reduction occurs by a high-speed jet air discharged from the enlarged pipe to cause a heat-insulated expansion to form a supersaturated atmosphere, with the result that water molecules in the atmosphere aggregate to form water molecular ions to be grown into large water cluster ions.
In the proposed ionization device, by carrying out a discharge in the inner space of the case, the ionization and the charging process to the molecules of the predetermined substance are executed in the same space.
Another ionization device has been proposed (see Patent Document 2) in which, in order to measure fine particles in an exhaust gas, a charging device is installed in the middle of an exhaust-gas introducing pipe, and a corona discharge is exerted inside the charging device so that fine particles in the exhaust gas passing therethrough are charged. In this ionization device also, the object fine particles to be ionized are directly charged in the charging device by the corona discharge.
Patent Document 1: Japanese Patent No. 3787773
Patent Document 2: JP-A No. 2005-24409
Non-Patent Document 1: J. Aerosol Sci., Vol. 16, No. 2, pp. 109-123, 1985